JO, NO! The placing of Johan Santana on the disabled list could be the tipping point that sends the suddenly-fragile Mets into a death spiral, says The Post’s George Willis. Photo: N.Y. Post: Charles Wenzelberg

R.a. Dickey came in to pitch the ninth inning for the Mets yesterday, and for a moment it looked as if the home team were poised to pull out the kind of dramatic come-from-behind win that had been its signature during the first half of the season.

But with two outs, the Dodgers’ Juan Uribe slammed a Dickey knuckleball over the left-field wall for a two-run homer to extend what was a one-run lead to a three-run bulge and an 8-5 victory at Citi Field, putting an exclamation point on another dark day for the Mets.

It began with the announcement Johan Santana had been placed on the 15-day disabled list to heal a sprained ankle and rest a fatigued arm. It ended with the team’s other ace giving up a two-run blast that dropped the team to .500 for the first time since May 4. The Mets have now lost eight of their past nine and seven of eight since the All-Star break. At 47-47, they are .500 for the first time since they were 13-13.

The loss on the field made the loss of Santana seem even more catastrophic, even though the Mets tried to downplay its significance. The spin manager Terry Collins and general manager Sandy Alderson offered was they anticipated there might come a time when Santana, coming off major shoulder surgery, might have to be shut down during the regular season to rest his arm and his body.

“I don’t think any of us should be surprised by this,” Alderson said. “Not that we expected it, necessarily. But I don’t think we should be surprised by this and the need to back him off.”

Still, it’s devastating news, coming at a time when the Mets desperately are trying to remain in contention for a division championship or a wild-card berth. The starting rotation already was jolted by the recent loss of Dillon Gee to a blood clot in his throwing shoulder. Plus, Mike Pelfrey made just three starts before having Tommy John surgery. Now the only Mets pitcher to throw a no-hitter is sidelined for at least two weeks, if not longer.

“They decided it was better for me to take some time off and that’s what we’re going to do,” Santana said.

The decision to put Santana on the DL came a day after he was shelled against the Dodgers on Friday night, when he gave up six runs in just three innings. The pitcher and the Mets brass insist there is no structural damage or pain in the surgically repaired shoulder. The ankle is the issue, they say. He initially injured it when it was stepped on while covering first base against the Cubs on July 6.

If all goes well, he should be back in the rotation sometime after the Mets return from their 11-game West Coast trip through Arizona, San Francisco and San Diego. The problem is the Mets, who currently view themselves as buyers for talent that could help in their playoff push, could find themselves with little chance of playing in October if their swoon continues.

“Realistically, the next seven, eight, 10 games become important to us,” Alderson admitted.

If the Mets continue their downward spiral, there won’t be any reason to be buyers, nor will there be any reason to rush Santana back until he’s 100 percent healthy. Right now, the Mets are trying to be survivors more than buyers.

Miguel Batista started yesterday for the fifth time this season. The 41-year-old was dreadful, throwing 81 pitches in just three innings, and allowed four runs on five hits with three walks.

Santana’s ankle coupled with Batista’s poor outing likely hastens the arrival of highly-touted prospect Matt Harvey. Alderson had said Harvey’s ascension to the major leagues would be “independent of our needs in 2012.” But the Mets may need him to salvage any hopes of making the playoffs.

The Mets keep saying they’re buyers when it comes to acquiring potential help for a bullpen that entered yesterday’s game with the worst ERA (4.95) in baseball. They are also in the market for a right-handed bat. But without Santana and without wins, the Mets may not have a reason to buy anything.